1. Field of the Invention
The present invention relates to an active noise eliminating system for eliminating residual sound noise within a closed space, such as a passenger room of automotive vehicle or aircraft, by actively generating sound for interference with the residual noise.
2. Description of the Prior Art
An example of the active noise eliminator is disclosed in British Published Patent Application No. GB-2149614, for instance, which is incorporated by reference herein. This noise eliminator comprises a plurality of microphones for detecting residual noise signals, a plurality of loud speakers for generating noise eliminating sound for interference with the residual sound noise, a signal processor for generating signals to the loud speakers, in response to residual noise signals detected by the microphones and the fundamental frequency of a noise source measured by fundamental frequency measuring means (incorporated in the signal processor), in such a way that the sound pressure level in the closed space can be minimized.
In this prior-art noise eliminator, four microphones and three loud speakers are arranged within the closed space. For simplification of the explanation, the assumption is made that only a single microphone and a single loud speaker are provided in a closed space. Under these conditions, the residual noise signal level E detected by the microphone can be expressed as EQU E=X.sub..rho. .multidot.H+X.sub..rho. .multidot.G.multidot.C
where
X.sub..rho. denotes the sound signal generated by a noise source; PA1 H denotes the transfer function from the noise source to the microphone; PA1 G denotes the transfer function required for noise elimination; and PA1 C denotes the transfer function from the loud speaker to the microphone.
In the above equation, if noise can be completely cancelled at the noise elimination point (the microphone position), EQU E=0 and therefore
G=-H/C
On the basis of the transfer function G thus obtained to minimize the noise signal level E detected by the microphone, filter coefficients of the signal processor are adaptively updated.
In the case when a plurality of microphones are arranged, LMS (least Mean Square) algorithm is adopted as a method of calculating the filter coefficients, by which the sum total of the noise signal levels En detected by a plurality of microphones can be minimized.
In the above-mentioned method of adaptively updating th filter coefficients in the signal processor, since the algorithm for obtaining the noise eliminating (minimizing) transfer functions G includes the transfer functions C between the loud speakers and the microphones, and additionally the transfer functions C is fixedly determined when the noise eliminator is shipped from a factory, the following problem arises: the transfer function C tends to vary due to changes in temperature in a closed space and/or in characteristics of the speakers and microphones with the passage of time, thus causing the convergent characteristics of the elimination algorithm to become unstable and further the sound pressure at the evaluation points to be inevitably increased into a divergent condition at the worst.